Publications in Scientific Journals:

S. Bashir, M. Rafique, W. Husinsky:
"Identification of ultra-fast electronic and thermal processes during femtosecond laser ablation of Si";
Applied Physics A: Materials Science & Processing, 109 (2012), 421 - 429.

English abstract:
Ultra-fast electronic and thermal processes for
the energy deposition mechanism during femtosecond laser
ablation of Si have been identified by means of atomic
force microscopy and Raman scattering techniques. For
this purpose, Si targets were exposed with 800-nm, 25-fs
Ti:sapphire laser pulses for different laser fluencies in air
and under UHV (ultra high vacuum) conditions. Various
nano- and microstructures on the surface of the irradiated
samples are revealed by a detailed surface topography analysis.
Ultra-fast electronic processes are dominant in the
lower-fluence regime. Therefore, by starting from the ablation
threshold three different fluence regimes have been chosen:
a lower-fluence regime (0.06-0.5 J cm−2 single-shot irradiation
under UHV condition and 0.25-2.5 J cm−2 singleshot
irradiation in ambient condition), a moderate-fluence
regime (0.25-1.5 J cm−2 multiple-shot irradiation), and a
higher-fluence regime (2.5-3.5 J cm−2 multiple-shot irradiation).
Around the ablation threshold fluence, most significant
features identified at the Si surface are nanohillocklike
structures. The appearance of these nanohillocks is regarded
as typical features for fast electronic processes (correlated
with existence of hot electrons) and is explained on
the basis of Coulomb explosion. The growth of these typical features (nanohillocks) by femtosecond laser irradiation
is an element of novelty. At moderate irradiation fluence,
a ring-shaped ablation with larger bumps and periodic
surface structures is observed and is considered as a
footprint of ultra-fast melting. Further increase in the laser
fluence, i.e. a higher-fluence regime, resulted in strong enhancement
of the thermal process with the appearance of
larger islands. The change in surface topography provides
an innovative clue to differentiate between ultra-fast electronic
processes, i.e. Coulomb explosion (sub-100 fs) at a
lower-fluence regime and ultra-fast melting (hundreds of fs)
at a moderate-fluence regime, and slow thermal processes
(ps time scale) at a higher-fluence regime. These fast electronic
and thermal processes are well correlated to structural
and crystallographic alterations, inferred from Raman spectroscopy.

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